Acoustic resonators can be used to implement signal processing functions in various electronic applications. For example, some cellular phones and other communication devices use acoustic resonators to implement frequency filters for transmitted and/or received signals. Several different types of acoustic resonators can be used according to different applications, with examples including surface acoustic wave (SAW) resonators and bulk acoustic wave (BAW) resonators.
A typical SAW resonator comprises a layer of piezoelectric material applied to substrate, and electrodes disposed on a surface of the piezoelectric layer. The resonators convert electrical signals to mechanical vibrations on a surface of the piezoelectric layer, and/or mechanical vibrations on the surface of the piezoelectric layer to electrical signals. A conventional SAW resonator may include an interdigital transducer (IDT) electrode disposed on the surface of the piezoelectric layer. The IDT electrode typically includes a first comb electrode comprising a first busbar and multiple first fingers extending from the first busbar, and a second comb electrode comprising a second busbar and multiple second fingers extending from the second busbar in an opposite direction, such that the first and second fingers provide an interleaving pattern. The piezoelectric layer of a SAW resonator is typically formed of lithium niobate (LiNbO3) (hereinafter “LN”) or lithium tantalate (LiTaO3) (hereinafter “LT”).
BAW resonators include thin film bulk acoustic resonators (FBARs), solidly mounted resonators (SMRs), stacked bulk acoustic resonators (SBARs), double bulk acoustic resonators (DBARs), and coupled resonator filters (CRFs), for example. A typical BAW resonator comprises a layer of piezoelectric material sandwiched between two plate electrodes in a structure referred to as an acoustic stack. Where an input electrical signal is applied between the electrodes, reciprocal or inverse piezoelectric effect causes the acoustic stack to mechanically expand or contract depending on the polarization of the piezoelectric material. As the input electrical signal varies over time, expansion and contraction of the acoustic stack produces acoustic waves (or modes) that propagate through the acoustic resonator in various directions and are converted into an output electrical signal by the piezoelectric effect. Some of the acoustic waves achieve resonance across the acoustic stack, with the resonant frequency being determined by factors such as the materials, dimensions, and operating conditions of the acoustic stack. These and other mechanical characteristics of the acoustic resonator determine its frequency response.
The piezoelectric layer of a BAW resonator is typically formed of aluminum nitride (AlN), zinc oxide (ZnO), or zirconate titanate (PZT), for example. Also, the piezoelectric layer may be “doped” with one or more rare earth elements, such as aluminum nitride (AlN) doped with scandium (Sc) to provide aluminum scandium nitride AlScN (hereinafter “ASN”), for example. LN or LT with c-axes of symmetry aligned vertically are typically not used as the piezoelectric material in conventional BAW resonators due to unacceptably low coupling coefficients kt2 and piezoelectric coupling coefficients e33 in communication bands having high RF frequencies (e.g. above 1 GHz). However, the materials used for piezoelectric layers in BAW resonators at high RF frequencies are not particularly efficient at lower frequencies, and require large resonator areas. This is inconsistent with the general design goal of providing smaller devices incorporating BAW resonators.